The present invention relates to the field of solar energy collection systems. More specifically, the present invention relates to a stable structure for supporting an array of photovoltaic modules of a tilted single-axis tracking solar energy collection system.
Solar energy collection systems are used for a variety of purposes, for example, as utility interactive power systems, power supplies for remote or unmanned sites, and cellular phone switch-site power supplies. An array of energy conversion modules, such as, photovoltaic (PV) modules, in a solar energy collection system can have a capacity from a few kilowatts to a hundred kilowatts or more, depending upon the number of PV modules, also known as PV panels, used to form the array, and can be installed wherever there is a reasonably flat area with exposure to the sun for significant portions of the day.
In general terms, a solar energy collection system has an array of PV modules arranged in the form of rows and mounted on a structure. The PV modules are oriented to optimize the PV module energy output to suit the particular PV system design requirements. PV modules may be mounted on a fixed structure, with a fixed orientation and fixed tilt, or may be mounted on a tracking structure. The tracking structure generally includes a rotatable support structure, which supports the PV modules and rotates on one or more specific axes. The tracking structure further generally includes one or more drive mechanisms that rotate the support structure around the one or more axes, either continuously or on an intermittent basis, to aim the PV modules toward the sun as the sun moves across the sky during the day and as the sun path moves in the sky during the year.
In solar energy collection systems, tracking the sun can lead to a significant increase in annual radiation falling on the tracked surface, thus an increase in efficiency, relative to a fixed structure. One such apparatus is a tracking structure that reorients the PV modules by mechanical tracking on two axes. The two-axis tracking solar energy collection structure allows the PV modules to face directly toward the sun regardless of the daily movement of the sun and the seasonal variation in the path of that movement. However, the structure for a two-axis system is more complex, costly, and prone to breakdown than a single-axis tracking solar energy collection structure.
A single-axis tracking solar energy collection structure represents a reasonable compromise between the fixed structure and the two-axis structure. That is, a single-axis tracking structure achieves the benefit of an increase in efficiency over a fixed structure without the undesirable complexity and cost of a two-axis tracking structure.
A single-axis tracking structure moves the PV modules around a single axis, and therefore approximates tracking of the actual position of the sun at any time. Usually, the rows of PV modules are arranged with their axes disposed in a north-south direction, and the drive mechanism gradually rotates the rows of PV modules throughout the day from an east-facing direction in the morning to a west-facing direction in the afternoon. The rows of PV modules are brought back to the east-facing orientation for the next day. A single-axis tracking structure may rotate around an axis that is either horizontal or tilted on an angle relative to horizontal that corresponds to the latitude of the location. However, tilted single-axis tracking structures generally achieve a performance that is improved relative to horizontal single-axis tracking structures because they place the array of PV modules on average closer to perpendicular relative to the path of the sun.
A particular type of tilted single-axis tracking structure makes use of the concept of polar axis tracking, utilized extensively in satellite antennae and telescope mounting. A polar axis tracking structure orients the PV modules toward the sun by rotating around an axis that is parallel to the axis of rotation of the earth. Polar single-axis tracking structures come closest to achieving the performance of two-axis tracking structures without the complexity and cost associated with tracking a second axis.
Polar single-axis tracking structures and other tilted single-axis tracking structures have been manufactured for many years. Unfortunately, many of the prior art structures suffer from a variety of problems. For example, some tilted single-axis tracking structures are designed for mounting on a pole that is normally embedded in the ground. A problem with such an approach is the structural inefficiency of a single-point pole support. The single point attachment leads to high bending stresses in the support structure and pole and is not very stable in elevated wind conditions.
The structural inefficiencies of the prior art devices under wind conditions is exacerbated by the trend toward solar energy collection systems having solar collection capacities in excess of three kilowatts. A capacity of greater than three kilowatts is a convenient size for many applications, such as, in a single residence application. However, to meet such capacities, the size of the array of PV modules has increased relative to earlier systems. Unfortunately, the single point attachment of the single pole systems has difficulty in supporting the increased size and weight of the larger arrays under wind load.
Other prior art single-axis tracking structures employ a tripod-like structure for supporting the PV modules. The structure includes a single footing and an A-frame. A torque tube is balanced by the single footing at one end of the tube, and the A-frame at the other end of the tube. PV modules are supported by and rotate about the torque tube. Unfortunately, this tripod structure requires three independent formal foundation elements, thus increasing installation complexity and consequently, cost. In addition, the independent nature of the structure legs and foundations leads to an undesirable transfer of a variety of structural loads to the ground. As such, winds and other weather phenomena can cause enormous torque loads on the structure and result in a loss of stability of the tracking structure, possibly leading to failure of the structure and/or drive mechanism.
In an attempt to counter the loss of stability and to accommodate larger PV arrays, the structure, foundation elements, drive mechanism, and so forth have increased in complexity, size, and weight. A heavy structural steel pedestal, typically embedded in a large concrete base or foundation, is needed to withstand the loads on the above ground structure. Typical installations have become sufficiently large so that cranes are required to move and install the structural steel, cement is trucked in to support the steel framework, and multiple visits to the site by multiple workers are required to complete the installation. Unfortunately, the construction of such a large structure is quite expensive and difficult in the typical remote locations where such systems are most advantageous.
Accordingly, it is an advantage of the present invention that a structure for supporting energy conversion modules is provided.
It is another advantage of the present invention that a structure is provided having a geometry that leads to an efficient transfer of structural loads to the ground.
Another advantage of the present invention is that a structure is provided that is stable in wind and other weather phenomena.
Yet another advantage of the present invention is that a structure is provided that is readily installed in a single site visit with conventional equipment, and requiring little site preparation and excavation.
The above and other advantages of the present invention are carried out in one form by a structure for supporting an energy conversion module above a surface. The structure includes a frame having a first leg, a second leg, and a base tensioning member. The first and second legs are configured to extend upwardly from the surface and join at an apex, and the base tensioning member is interposed between the first and second legs. A torque tube is pivotally retained by the frame at the apex and is configured for attachment of the energy conversion module. A foot member rotatably retains a tube end of the torque tube and is configured to contact the surface. A first tensioning member is coupled between the foot member and the first leg, and a second tensioning member is coupled between the foot member and the second leg. A pre-tensioning member extends from the apex of the frame and is configured for attachment to the surface.
The above and other advantages of the present invention are carried out in another form by a structure for supporting an energy conversion module above a surface. The structure includes a frame having a first leg, a second leg, and a base tensioning member. The first and second legs are configured to extend upwardly from the surface and join at an apex, and the base tensioning member is interposed between the first and second legs. A torque tube is pivotally retained by the frame at the apex and is configured for attachment of the energy conversion module. A foot member rotatably retains a tube end of the torque tube and is configured to contact the surface. A first tensioning member is coupled between the foot member and the first leg, and a second tensioning member is coupled between the foot member and the second leg. The structure further includes pre-tensioning members configured for attachment to the surface, one each of the pre-tensioning members extending from one each of the first leg, the second leg, and the foot member.